1. Field of the Invention
The invention relates to isocyanate-free foamable mixtures comprising prepolymers and a hydrocarbon blowing agent.
2. Description of the Related Art
Sprayable in-situ foams are employed for filling hollow spaces, in particular in the building sector. Here, they are used, inter alia, for sealing joins, e.g. around windows and doors, and act as excellent insulating materials so as to give good thermal insulation. Further applications are, for example, insulation of pipes or filling hollow spaces in industrial appliances with foam.
All conventional in-situ foams are polyurethane foams (PU foams) which in the uncrosslinked state comprise prepolymers which have a high concentration of free isocyanate groups. These isocyanate groups are able to undergo addition reactions with suitable reaction partners even at room temperature, as a result of which curing of the spray foam is achieved after application. The foam structure is produced by incorporating a volatile blowing agent into the as yet uncrosslinked raw material and/or by means of carbon dioxide formed by reaction of isocyanates with water. The foam is generally ejected from pressure cans by means of the autogenous pressure of the blowing agent.
Reaction partners employed for the isocyanates are alcohols having two or more OH groups, especially branched and unbranched polyols, or else water. The latter reacts with isocyanates to liberate carbon dioxide, as mentioned above, and form primary amines which can then add directly onto a further, as yet unconsumed isocyanate group. This results in formation of urethane and urea units which, owing to their high polarity and their ability to form hydrogen bonds in the cured material, can form partially crystalline substructures and thus lead to foams having a high hardness, pressure resistance and ultimate tensile strength.
Blowing agents used are mostly gases which are condensable at a relatively low pressure and can thus be mixed in the liquid state into the prepolymer mixture without the spray cans having to be subjected to excessively high pressures. In addition, the prepolymer mixtures contain further additives such as foam stabilizers, emulsifiers, flame retardants, plasticizers and catalysts. The latter are usually organic tin(IV) compounds or tertiary amines. However, iron(III) complexes, for example, are also suitable here.
PU spray foams are produced both as one-component (1K) foams and as two-component (2K) foams. The 1K foams cure exclusively through contact of the isocyanate-containing prepolymer mixture with atmospheric moisture. Foam formation can additionally be aided by the carbon dioxide liberated during the curing reaction of 1K foams. 2K foams comprise an isocyanate component and a polyol component which have to be mixed well with one another immediately before foaming and cure as a result of the reaction of the polyol with the isocyanates. An advantage of the 2K systems is an extremely short curing time. Thus, it sometimes only takes a few minutes for a 2K foam to cure to such an extent that it can be cut. However, 2K foams have the disadvantage that they require a complicated pressure can having two chambers and, in addition, are significantly less comfortable to handle than the 1K systems. The latter in particular has resulted in 1K foams being used considerably more frequently than 2K foams in Europe.
The cured PU foams display, in particular, excellent mechanical and thermal insulation properties. Furthermore, they have very good adhesion to most substrates and are stable virtually indefinitely under dry and UV-protected conditions. Further advantages are the toxicological acceptability of the cured foams from the point in time at which all isocyanate units have reacted quantitatively, and their swift curing and their easy handling. Due to these properties, PU foams have been found to be very useful in industrial practice.
A further important advantage of PU foams is the fact that their burning behavior can relatively easily be improved significantly by addition of flame retardants. Thus, for example, burning class B2 (normal combustibility) can be achieved without problems in the case of PU foams using customary flame retardants. In some countries, e.g. Germany, this burning class is prescribed by law for in-situ foams. As flame retardant, use is made, in particular, of tris(chloro-propyl) phosphate, usually in the form of technical-grade mixtures of the various regioisomers. This flame retardant is present virtually without exception in commercially available in-situ PU foams having improved burning behavior. The most important reason for this widespread use of tris(chloropropyl) phosphate is the fact that among liquid flame retardants there is at present no inexpensive alternative having comparable effectiveness. The often very effective solid flame retardants, on the other hand, are unsuitable for use in in-situ PU foams since they can block the valve during foaming. In particular, however, solid flame retardants can jam in the valve during the foaming procedure, so that the valve subsequently no longer closes, which leads to uncontrolled complete emptying of the foam can.
A disadvantage of tris(chloropropyl) phosphate as most widely used flame retardant for in-situ PU foams is the fact that it is not reactive and is therefore not incorporated into the network formed during curing of the foam. Although there is generally no risk of subsequent release of the unbound flame retardant in the case of in-situ foams because these foams are closed-cell and, in addition, are usually covered with plaster or render, this flame retardant reduces the network density as a result of the absence of chemical bonding to the PU polymer network and thus acts as plasticizer. Nevertheless, PU foams have, in this respect, the advantage that their hardness depends mainly on the concentration of the PU and urea units present in them. Thus, in particular, their ability to form hydrogen bonds is responsible for the good mechanical properties of PU foams. The plasticizing action of the flame retardant can therefore be compensated relatively readily by appropriate adaptation of the PU and urea group density in the cured foam. Since the PU and urea units are to a large measure formed only during curing of the foam and thus have no influence on the viscosity of the foamable mixture before curing, this measure can be employed without problems.
Despite all their advantages, PU spray foams have the critical disadvantage that the isocyanate groups can, owing to their high reactivity, also develop a serious irritant action and toxic effects. In addition, the amines which can be formed by reaction of monomeric diisocyanates with an excess of water are in many cases suspected of being carcinogenic. In fact, carcinogenic action has already been detected. Such monomeric diisocyanates are likewise present in addition to the isocyanate-terminated prepolymers in most spray foam mixtures. The uncrosslinked spray foam compositions are thus toxicologically unacceptable until they are completely cured. Critical factors here are not only direct contact of the prepolymer mixture with the skin but also, in particular, possible aerosol formation during application of the foam or vaporization of low molecular weight constituents, e.g. monomeric isocyanates. This results in the risk of toxico-logically unacceptable compounds being taken up via inhaled air. In addition, isocyanates have a considerable allergenic potential and can, inter alia, trigger asthma attacks. These risks are increased by the fact that the PU spray foams are often not used by trained and practiced users but by handymen and home workers, so that correct handling cannot always be assumed.
The hazard potential exhibited by conventional PU foams and the associated compulsory labeling has additionally resulted in the problem of considerably decreasing acceptance of the corresponding products by users. In addition, completely or partly emptied spray cans are classified as hazardous waste and have to be labeled accordingly and in some countries, e.g. Germany, even have to be made available for reuse by means of a costly recycling system.
In order to overcome these disadvantages, DE-A-43 03 848, inter alia, has described prepolymers for spray foams which contain no monomeric isocyanates or contain only low concentrations of these. However, a disadvantage of such systems is the fact that the prepolymers always still have isocyanate groups, so that such PU spray foams may well be better than conventional foams from a toxicological point of view but cannot be described as nonhazardous. In addition, the acceptance and waste problems are not solved by such foam systems.
It would therefore be desirable to have prepolymers which do not crosslink via isocyanate groups and are thus toxicologically acceptable available for the production of spray foams. Moreover, these prepolymer mixtures should make it possible to produce spray foams which in the cured state have similarly good properties and, in particular, a comparable hardness compared to conventional isocyanate-containing PU foams. In addition, one-component spray foam systems which cure exclusively through contact with atmospheric moisture also have to be possible. These should display comparably problem-free handling and processibility including a high curing rate even at a low catalyst concentration. The latter is important particularly since the organotin compounds generally used as catalysts are likewise problematical from a toxicological point of view. In addition, tin catalysts often also contain traces of highly toxic tributyltin derivatives. It would therefore be particularly advantageous to have a prepolymer system which has such favorable curing properties that a tin catalyst can be entirely dispensed with.
On this subject, the literature, e.g. US-A-6020389, describes condensation-crosslinking silicone foams which comprise alkoxy-, acyloxy- or oximo-terminated silicone prepolymers. Such foamable mixtures are in principle suitable for producing 1K foams which cure at room temperature only through atmospheric moisture. However, such systems comprising purely silicone-containing prepolymers can be used only for producing elastic flexible to semi-rigid foams. They are not suitable for producing rigid, nonbrittle in-situ foams. EP-1098920-A, DE-10108038-A and DE-10108039-A describe prepolymer mixtures comprising alkoxysilane-terminated prepolymers for producing rigid spray foams. These are polymers having an organic backbone which generally has a conventional polyurethane structure. In EP-1098920-A and DE-10108038-A, this organic backbone is formed by reaction of customary diisocyanates with polyols. Here, an appropriate excess of diisocyanates is used so that isocyanate-terminated prepolymers are obtained. These can then be reacted with 3-aminopropyltrimethoxysilane derivatives in a second reaction step to form the desired alkoxysilane-terminated polyurethane prepolymers. In DE-10108038-A, a specific reactive diluent is added to the silane-terminated prepolymers. DE-10108039-A describes a second process for preparing alkoxysilane-terminated prepolymers, in which these prepolymers are formed by reaction of hydroxy-functional polyols with 3-isocyanatopropyltrimethoxy-silane.
These alkoxysilane-terminated prepolymers and any reactive diluents present can condense with one another in the presence of a suitable catalyst and of water with elimination of methanol and as a result cure. The water can be added as such or can originate from contact with atmospheric moisture. Both 1K and 2K foams can thus be produced using such a system.
However, the alkoxysilane-terminated polyurethane prepolymers described in EP-1098920-A, DE-10108038-A and DE-10108039-A have, inter alia, the disadvantage of a relatively low reactivity toward atmospheric moisture. For this reason, high concentrations of a tin catalyst are necessary to achieve sufficiently rapid curing.
A significant improvement is provided by a system described in WO 02/066532. The alkoxysilane-terminated prepolymers described here for producing isocyanate-free spray foams comprise silane end groups of the general formula [1]
where:    X and Y are each an oxygen atom, an N—R′ group or a sulfur atom,    R1 is an alkyl, cycloalkyl, alkenyl or aryl radical having 1-10 carbon atoms,    R2 is an alkyl radical having 1-2 carbon atoms or an ω-oxaalkylalkyl radical having a total of 2-10 carbon atoms,    R′ is a hydrogen atom, an alkyl, alkenyl or aryl radical having 1-10 carbon atoms or a —CH2—SiR1z(OR2)3-z group and z is 0 or 1,with the proviso that at least one of the two groups X and Y is an NH function.
In these alkoxysilyl-terminated prepolymers, the crosslinkable alkoxysilyl groups are separated from a urethane or urea unit only by one methyl spacer. These prepolymers are astonishingly reactive toward water and thus have extremely short tack-free times in the presence of atmospheric moisture and can even be crosslinked in the absence of tin.
Two further critical disadvantages of silane-terminated prepolymers for spray foam applications could, on the other hand, be overcome in none of the patents or patent applications mentioned. Thus, all silane-crosslinking foams described in the prior art without exception display very ready combustibility. It is not possible to achieve a significant improvement in the burning behavior by the addition of flame retardants such as tris(chloropropyl) phosphate since, unlike in the case of conventional PU foams, the plasticizing action of these flame retardants cannot be compensated, or be compensated only unsatisfactorily, in the case of foams comprising alkoxysilane-terminated prepolymers. In this case, all PU and urea units which play a critical role in achieving the hardness of the resulting foam are already present in the prepolymer and have an effect on its viscosity. The concentrations of these groups can therefore be increased only to a very limited degree. As a consequence, the addition of relatively large amounts of flame retardants to silane-crosslinking spray foams according to the prior art leads without exception only to very soft, unstable foams. On the other hand, lower concentrations of flame retardants produce no appreciable effect.
The second disadvantage relates to the fact that cracks can form in known silane-crosslinking foams under certain conditions. This crack formation is particularly pronounced when the foam is foamed in a model join as shown in FIG. 1 whose wooden boards have been moistened beforehand. This crack formation is attributable to the polar blowing agents used in the prior art. This is because the diffusion of these polar blowing agents through the foam lamellae, which are likewise composed of polar material, proceeds significantly more quickly than the diffusion of nonpolar air occurring in the opposite direction. This can lead to shrinkage and subsequently even rupture of the only partially cured and thus not sufficiently cracking-resistant foam, because, unlike in the case of conventional PU foams, curing does not result in liberation of carbon dioxide which could compensate the blowing agent shrinkage until curing of the foam is concluded.
Crack formation can be avoided if nonpolar blowing agents such as propane/butane-containing mixtures are used, since nonpolar blowing agents diffuse significantly more slowly through the foam lamellae, so that the foam displays a considerably reduced tendency to shrink and to form cracks. However, a disadvantage of this measure is the fact that nonpolar blowing agents such as propane/butane are incompatible with the silane-terminated prepolymer. Although foamable emulsions can be produced using known prepolymers and propane/butane, these are not stable on storage and can no longer be foamed after demixing has occurred. Owing to the high viscosity of the silane-terminated prepolymers at room temperature, reemulsification is likewise not possible. For this reason, nonpolar blowing agents such as propane/butane can only be used in mixtures with other, more polar blowing agents. However, the known silane-crosslinking foams which then result are once again not crack-free in the moistened model join as shown in FIG. 1.
A criterion which can be used for a sufficient improvement in the burning behavior is spontaneous extinguishing of a vertical foam surface after brief application of a flame (5 s) to its bottom edge by means of a Bunsen burner, as shown in FIG. 2.
This combination of properties, viz. improved burning behavior and absence of cracks, means that numerous measures which in the case of conventional PU foams lead to a significant improvement in the burning behavior cannot be applied to alkoxysilane-crosslinking foams. In this context, mention may be made, for example, of the replacement of tolylene diisocyanate (TDI), which is present in silane-crosslinking foams corresponding to the prior art, by less combustible diphenylmethane diusocyanate (MDI) or polymeric MDI (p-MDI). Thus, the use of p-MDI as polyisocyanate leads to silane-terminated prepolymers which are totally incompatible with all customary blowing agents. Foaming of these prepolymers is not possible. Although the silane-terminated prepolymers obtained using MDI as diisocyanate display improved blowing agent solubility, they, too, are compatible only with blowing agent mixtures which consist essentially of polar blowing agents such as 1,1,1,2-tetrafluoroethane or 1,1-difluoroethane. It is in this case not possible to obtain foamable mixtures which remain crack-free in the moistened model join as shown in FIG. 1. For the same reason, it is likewise impossible to replace, completely or partly, the readily combustible polyether polyols present in foamable silane-terminated prepolymers according to the prior art by less combustible aromatic or aliphatic polyester polyols.